It occurs to me that while Tarquin's priorities prior to his arrival in the story were to live as long as possible and gain as much power as possible it is entirely plausible that upon meeting Elan his goals have changed. I suspect that his defeat will not come at the point of a sword. Nor will it come from a revolution or any other such violent means. It will come from his favoured son looking him in the eyes and saying:

: Dad, I feel sorry for you.

And just walking away.

He has displayed a desire to interact with Elan, to become a part of his life it appears to be part of his new agenda. If that's taken from him I doubt he'd handle it well. He might find that the thrill has gone out of tyranny and life loses it's purpose. (Insert a series of platitudes about T's life becoming pointless and him just walking into the desert one night after a glass of wine.)